Quick background: I have an 03 Taurus Vulcan with now 180k miles (shocking) and I'm trying to diagnose a mysterious misfire that keeps coming back. I've gone through 3 sets of [Motorcraft] plugs in the past year, all were properly gapped and torqued. I started out with a cylinder 2 misfire, then cylinder 5, now cylinder 4. I'm running all Motorcraft parts (coilpack, wires, plugs), and I recently replaced the upper & lower intake gaskets, so now I'm convinced that it could be something deeper than that. When I plug in my OBD scanner, I get a p0304 for the misfire and a pending p0316 for misfire within first 1k revolutions. A majority of the time if I baby it, it will drive with no issues, until I shut it off. When I restart it, it's fine for about 15 seconds then you can slowly feel the misfire set in, and with time it gets worse til the light starts to flash. Keep in mind it only does this when at operating temp. I'm thinking the plugs might be running too hot, or a possible loss of compression in the #4 cylinder, mainly because of the fact that the misfire is down near idle (correct me if I'm wrong).
NOW the fun part, for those who have knowledge on fuel trim: I've read that the '96-2000s should be between 37 and 45psi, and 2000+ should be about 45 to 56psi. I get readings anywhere from a minimum of 36.52psi and a maximum of 43.04psi, mainly averaging between 39-41, mostly at 40 on the dot. My questions are: whether it be idling or cruising, is the fuel pressure reading supposed to fluctuate or remain steady? Also for the STFT Banks 1 & 2, only S2 (for both banks) I get a reading of 99.22% all the time. Is this normal? And lastly, which is more accurate for diagnosing a fuel problem, a steady closed-loop idle or steady cruising speed?
Any input would be greatly appreciated, thanks guys in advance.